Types defined with type_alias_impl_trait can be used for dispatch in trait impls. Which is unsound.

[steffahn]

Let’s define a type:

trait Foo {}
impl Foo for () {}
type Bar = impl Foo;
fn _defining_use() -> Bar {}

Well, a straightforward trait implementation does not work:

trait Trait {}

impl Trait for Bar {}

   Compiling playground v0.0.1 (/playground)
error: cannot implement trait on type alias impl trait
  --> src/lib.rs:10:1
   |
10 | impl Trait for Bar {}
   | ^^^^^^^^^^^^^^^^^^
   |
note: type alias impl trait defined here
  --> src/lib.rs:5:12
   |
5  | type Bar = impl Foo;
   |            ^^^^^^^^

error: aborting due to previous error

(playground)

But rustc doesn’t complain about anything slighty more complex at the moment, e.g.

trait Trait {}
impl Trait for (Bar,) {}

trait TraitArg<T> {}
impl TraitArg<Bar> for () {}

(playground)

Which turns out to be problematic:

trait TraitArg<T> {
    fn f();
}

impl TraitArg<Bar> for () {
    fn f() {
        println!("ho");
    }
}

fn main() {
    <() as TraitArg<Bar>>::f();
}

   Compiling playground v0.0.1 (/playground)
thread 'rustc' panicked at 'called `Result::unwrap()` on an `Err` value: ErrorReported', compiler/rustc_mir/src/monomorphize/collector.rs:827:84
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.53.0-nightly (727d10156 2021-04-27) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: internal compiler error: Encountered error `Unimplemented` selecting `Binder(<() as TraitArg<()>>, [])` during codegen
  |
  = note: delayed at compiler/rustc_trait_selection/src/traits/codegen.rs:68:32

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:1018:13
stack backtrace:
   0:     0x7f995e14cfd0 - std::backtrace_rs::backtrace::libunwind::trace::hdcf4f90f85129e83
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/../../backtrace/src/backtrace/libunwind.rs:90:5
   1:     0x7f995e14cfd0 - std::backtrace_rs::backtrace::trace_unsynchronized::h2669e30cb82f6732
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f995e14cfd0 - std::sys_common::backtrace::_print_fmt::hfbda19e17f6db318
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7f995e14cfd0 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h1a8751bf59281272
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/sys_common/backtrace.rs:46:22
   4:     0x7f995e1be31f - core::fmt::write::h7aa6cd0067dca82a
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/core/src/fmt/mod.rs:1094:17
   5:     0x7f995e1418d5 - std::io::Write::write_fmt::hd7dd3a1df9b6befb
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/io/mod.rs:1584:15
   6:     0x7f995e150d2b - std::sys_common::backtrace::_print::h551e9ec8a9fa8106
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/sys_common/backtrace.rs:49:5
   7:     0x7f995e150d2b - std::sys_common::backtrace::print::ha4b1c5e95fa040b3
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/sys_common/backtrace.rs:36:9
   8:     0x7f995e150d2b - std::panicking::default_hook::{{closure}}::h0b34c9ab7fb9f857
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/panicking.rs:208:50
   9:     0x7f995e15080d - std::panicking::default_hook::h3067e8318decd17a
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/panicking.rs:225:9
  10:     0x7f995e91382d - rustc_driver::report_ice::hf065daa4796f245b
  11:     0x7f995e151440 - std::panicking::rust_panic_with_hook::h81b8facc50f34daa
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/panicking.rs:595:17
  12:     0x7f995e151017 - std::panicking::begin_panic_handler::{{closure}}::ha376ab85d95a000e
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/panicking.rs:497:13
  13:     0x7f995e14d48c - std::sys_common::backtrace::__rust_end_short_backtrace::h6795c8afdd1a77e6
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/sys_common/backtrace.rs:141:18
  14:     0x7f995e150f79 - rust_begin_unwind
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/panicking.rs:493:5
  15:     0x7f995e11cdbb - std::panicking::begin_panic_fmt::hf43a0025042538e2
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/panicking.rs:435:5
  16:     0x7f9960d9e029 - rustc_errors::HandlerInner::flush_delayed::h340bd563bba016cc
  17:     0x7f9960d9cb1b - <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop::h56e016ed68f55f61
  18:     0x7f9960406cd6 - core::ptr::drop_in_place<rustc_session::parse::ParseSess>::hf9273749d5982cb2
  19:     0x7f99604157e1 - <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop::h2bbea2bbb380f5f7
  20:     0x7f99603fba0d - core::ptr::drop_in_place<rustc_interface::interface::Compiler>::h01f06b96a4e05fdc
  21:     0x7f99603fb5e4 - rustc_span::with_source_map::hafa0f290d807fcb7
  22:     0x7f99603fd161 - rustc_interface::interface::create_compiler_and_run::h31928174eaa223b1
  23:     0x7f99604168e8 - scoped_tls::ScopedKey<T>::set::h332c2cb9377dc4c6
  24:     0x7f9960416e03 - std::sys_common::backtrace::__rust_begin_short_backtrace::h3f92abbf0f820c98
  25:     0x7f996041a625 - core::ops::function::FnOnce::call_once{{vtable.shim}}::h3affa87e0b1d695f
  26:     0x7f995e15fe57 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h3aa31cb6360b59d9
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/alloc/src/boxed.rs:1546:9
  27:     0x7f995e15fe57 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h7719d3c7c5841461
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/alloc/src/boxed.rs:1546:9
  28:     0x7f995e15fe57 - std::sys::unix::thread::Thread::new::thread_start::hfbe13ead469fd0bc
                               at /rustc/727d101561f9b1e81c6282943292d990288ca479/library/std/src/sys/unix/thread.rs:71:17
  29:     0x7f995e09c609 - start_thread
  30:     0x7f995dfb0293 - clone
  31:                0x0 - <unknown>

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.53.0-nightly (727d10156 2021-04-27) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
thread panicked while panicking. aborting.
error: could not compile `playground`

(playground)

---

Unsurprisingly, this is actually not only resulting in ICEs but also in unsoundness:

#![feature(min_type_alias_impl_trait)]

trait Foo {}
impl Foo for () {}
type Bar = impl Foo;
fn _defining_use() -> Bar {}

trait Trait<T, In> {
    type Out;
    fn convert(i: In) -> Self::Out;
}

impl<In, Out> Trait<Bar, In> for Out {
    type Out = Out;
    fn convert(_i: In) -> Self::Out {
        unreachable!();
    }
}

impl<In, Out> Trait<(), In> for Out {
    type Out = In;
    fn convert(i: In) -> Self::Out {
        i
    }
}

fn transmute<In, Out>(i: In) -> Out {
    <Out as Trait<Bar, In>>::convert(i)
}

fn main() {
    let d;
    {
        let x = "Hello World".to_string();
        d = transmute::<&String, &String>(&x);
    }
    println!("{}", d);
}

�@U

(playground)

@rustbot modify labels: A-typesystem, A-traits, A-impl-trait, F-type_alias_impl_trait, T-compiler, requires-nightly
and someone please add “I-unsound 💥”.

[lqd]

just noting that this issue and #86411 are likely duplicates